Magnetism Notes Lodestones were discovered 2000 years ago and were magnetic. They were named after Magnesia which is a region in Greece where they were found. The Chinese used them for navigating ships in the 12th century. We know they contained iron ore which is called magnetite. Hans Christian Oersted was a Danish professor who, in 1820, first discovered the relationship between electricity and magnetism. He noticed electric currents would deflect compass needles. This link was critical in allowing scientists to make the leap to electric power, radio and television. Magnetic Poles are regions that produce magnetic forces. Magnets exert forces on one another. - they attract and repel without touching - strength of forces between them depends on distance - Each magnet has a north and south seeking pole o north seeking poles point north o south seeking poles point south o unlike electric charges, magnetic poles cannot be isolated, meaning a north pole never exists without a south pole o this suggests that atoms themselves are magnets Magnetic field is the space around a magnet where magnetic force is exerted. Magnetic field lines reveal the shape of the field. The shape of the field is typically the lines curving away from one pole and back towards the magnet at the other pole, as in the image below: Draw a magnetic field around a rod magnet:

Magnetic field strength is greater at the poles and the direction of the field outside the magnet is from north to south. The nature of the magnetic field Magnetic fields are produced by the motion of electric charge. Atoms contain charged particles. Specifically, we will look at electrons within an atom. If the pairs of electrons in an atom spin in the same direction, a strong magnet is produced. If a pair of electrons spin in opposite directions, the magnetic fields cancel and the material is not magnetic. This electron pairing is what helps to determine if a material is magnetic or not. Magnetic domains occur when dipoles (atomic magnets) line up with their magnetic axis in the same direction. This occurs in substances that are ferromagnetic. Ferromagnetic materials are materials containing iron, nickel, and cobalt that can be induced to become magnetic. -

soft ferromagnetic – demagnetize spontaneously

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hard ferromagnetic – keep the dipoles aligned

Effects of the domain theory Magnetic induction - Iron becomes temporarily magnetized when near a permanent magnet - stroking can cause iron to act like a permanent magnet - Reverse magnetism – occurs when a magnet is placed in a stronger magnetic field of opposite polarity - Demagnetization – caused by dropping or heating - Breaking a bar magnet creates smaller magnets with identical dipole alignments - magnetic saturation – when the maximum number of dipoles are aligned - Keepers – soft iron used in the storage of bar magnets - the earth can induce magnetism when iron is heated or hammered while in the earth’s magnetic field Electric currents and magnetic fields A single moving charge can produce a magnetic field. So can many moving charges, as in electric current. Bending a wire in a loop intensifies the magnetic field (Solenoid). An electromagnet is formed by current carrying coils of wire with many loops. Types of electromagnetism 1. ferromagnetic – strong 2. paramagnetic – magnetize slightly 3. diamagnetic – causes decreases in magnetic fields Factors affecting magnetic fields of electromagnets include the current traveling through the wires, the number of loops in the coil and the type of core material in the electromagnet. The magnetism of the core is measured in Telsa. Electromagnets are used for many purposes. Some of these include as lifting magnets, electromagnetic relay switches, electric bells, particle acceleration, MRI and the mag-lev trains Magnetic forces on moving charged particles Magnetic forces on a moving charged particle are greatest when a particle moves perpendicular to the magnetic field lines. At any other angle, the force is less. When it moves parallel to the magnetic field lines, the magnetic force is zero. The earth’s magnetic field deflects cosmic radiation which is good or else the radiation would be too intense for us to handle. These forces are also used to spread electrons on inner surface of TV tubes (which is the old TV’s). Magnetic forces on current carrying wires A wire carrying charged particles through a magnetic field is deflected. Speakers work because of a current carrying wire deflects a magnet attached to the speaker. Meters to motors

Galvanometer is a sensitive device used to measure the magnitude and direction of small electric currents. Galvanometers can be calibrated to detect current which would make it an ammeter. It can also be calibrated to measure electrical potential which would make it a voltmeter. An electric motor is a device that converts electric potential energy into mechanical kinetic energy Some of the parts in an electric motor include: 1. commutator – split copper ring 2. brushes – contacts 3. rotor – moving part 4. stator – non moving part Speed of the motor depends on: 1. current 2. magnet strength 3. number of coils 4. size of load attached 5. permeability of armature Earth’s magnetic field (William Gilbert) The earth is basically one huge magnet. However, the north and south poles are not located exactly at the locations we call the north and south poles. The angle between the geographic (true) North Pole according to the earth’s magnetic field and the magnetic north that compasses point to is called magnetic declination. Magnetic north has wandered throughout geologic time. More than 20 reversals have been recorded in the orientation of the dipoles in rocks on earth (over 5 billion years!) Magnetic inclination (dip) is the angle between the direction of the magnetic field and a horizontal line. It shows the direction towards magnetic north, including relative to the horizon. It will point towards the earth. It is measured with a dipping needle. Electromagnetic Induction Michael Faraday and Joseph Henry both discovered that electric current could be produced in a wire by simply moving a magnet in and out of a wire coil. (Law of Magnetic Induction) Electromagnetic induction occurs when voltage is induced by changing the magnetic field around a conductor (Heinrich Lenz) Faraday’s Law The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops. The amount of voltage produced by magnetic induction depends on: - how quickly the magnetic field lines are traversed by the wire - number of loops in the wire - strength of the magnetic field

The amount of current produced by magnetic induction depends on: - induced voltage - resistance of the coil - circuit to which it is connected Generators and alternating current The frequency of the induced alternating voltage equals the frequency of the changing magnetic field within the loop. What this means is that the more frequently the coil and the magnet are moved away and towards each other, the greater the induced voltage becomes. It is best accomplished by rotating a coil in a stationary magnetic field. A generator is a device that converts mechanical energy in a magnetic field into electrical energy (basically the opposite of a motor which converts electrical energy into mechanical energy). Voltage induced by a generator alternates and the current changes magnitude and direction periodically. In North America it changes magnitude and direction 60 times per second (in other words, at 60 Hz). How is Alternating current (AC) produced? 1. Huge coils of many loops are wrapped around an iron core 2. Iron core is connected to an assembly of paddle wheels 3. Some kind of energy source drivers the turbine 4. Electric energy is produced Transformers are simple devices capable of changing electric potentials The parts of a transformer include: - Primary coil – where current is supplied - secondary coil – where current is induced Types of transformers: - Step-up transformer – a transformer with more loops or turns on the secondary coil. It increases voltage - Step-down transformer – a transformer with fewer loops on the secondary coil. It reduces voltage.

Formula:

V1 V2  N1 N 2

Where V1 = primary voltage, V2 = secondary voltage N1 = # of primary turns, N2 = # of secondary turns

Ideal transformer – assumed to be 100% efficient, but most are only 96%. This is a transformer that increases voltage and decreases current.

V1 I1  V2 I 2

Power Transmission Voltage is stepped up to 120,000 volts or more when it leaves the power plant but the current stays low to prevent energy loss to heating the wires. Cities step down the voltage to around 2200V. Pole transformers finally reduce the voltage to 120V to be used in household circuits. Electromagnetic waves James Clerk Maxwell introduced the ideas that lead to understanding electromagnetic waves. 1. a changing electric field will create a changing magnetic field 2. they move perpendicular to each other at the speed of light 3. no medium is required An electromagnetic wave is composed of vibrating electric and magnetic fields that regenerate each other. Examples include visible light and all of the electromagnetic spectrum. Visible light is produced at a frequency to which our eyes are sensitive.